Method of deposition of silicon by using pyrolysis of silane

ABSTRACT

An improved method of deposition of silicon by using pyrolysis of silane, which employs a carrier gas of a mixture of hydrogen gas and an inert gas such as argon, helium, neon, krypton, xenon, radon and nitrogen.

United Stat es Patent 1191 Hara [4 June 28, 1974 METHOD OF DEPOSITION OF SILICON BY USING PYROLYSIS OF SILANE [56] References Cited, 1 [75] Inventor: Tohru Hara, Osaka, Japan UNITED STATES PATENTS 3,490,961 l/l970 Frieser et all 117/106 [73] Asslgnee' g jxgz s' i fi g' gifg s 3,653,991 4/1972 Sirtl et al. 117/106 p 3,669,769 6/l972 Baoami et a] i 17/106 [22] Filed: Dec. 27, 1971 Primary Examiner- Douglas J. Drummond [2]] Appl' 211328 Assistant Examiner-J. Massie [30] Foreign Application Priority Data [57] ABSTRACT 1970 Japan 45424747 An improved method of deposition of silicon by using Dec. 29, 1970 Japan 45423814. py y of Silane, wh p y a carrier g of a mixture of hydrogen gas and an inert gas such as ar- 117/106 117/1072 eg gon, helium, neon, krypton, xenon, radon and nitro- [58] Field of Search 117/106, 107, 107.2 R,

1 1l7/l35.1 I Ilaim, 7 Drawing Figures I I 1 i PATENTEDJUH28 lam SHEET 1 [IF 7 Fig.

A Y n 18 A #8 16 I4 INVENTOR T Ru HA A BY PATENTED v 3,821,020

SHEET [1F 7 6 O v g o O Q O 0 0 2 g I:

03 CD LO D N3 0 INVENTOR T HRU H RA BY m PATENTEUmzs 074 3,821,020

sum 6 OF 7 INVENTOR M 11 RA PATENTEDJUH28 1w 3; 821 020 I 800 900 I000. ||00 I200 TEMPERATURE (C) INVENTOR T K HAKA BY g zy METHOD OF DEPOSITION OF SILICON BY USING PYROLYSIS OF'SILANE This invention relates to a method for producing a silicon wafer and more particularly to a method for epitaxially growing silicon crystals by using pyrolysis of silane.

, respectively represents the temperature T and the decomposition rate a.

If, in this instance, the activity of the solid is assumed to be unity, then the equilibrium constant K of the reacformer. Furthermore, the latter is suitable for producing a silicon epitaxial wafer especially with a steep with uniform impurity concentration, because the carrier gas in the process of the method does not include gaseous chlorides which invites the undesired vapour etching and auto-doping. v

The method however is not fully acceptable because the method still necessitates so a high temperature that the property of the resultant silicon wafer does not completely meet the requirements of high-frequency or low-noise transistors.

It is therefore an object of this invention to provide improved method of growing silicon crystals by using pyrolysis of silane, at a relatively low temperature.

In the drawings FIG. 1 is a sectional view of a growth apparatus employed for the method of this invention;

FIG. 2 is a graphic illustration showing variations of growth rate ofsilane in terms of temperature;

FIGS. 3 through 6 are graphic illustrations showing variations of ratio of decomposed silane to hydrogenargon mixed carrier gas; and

FIG. 7 is a graphic illustration showing variations of ratio of decomposed silane to carrier hydrogen gas.

In FIG. 1, a vertical type reaction apparatus which is generally used in the growth of silicon is illustrated, which includes a quartz bell-jar reactor 10 having a gas feed tube 12 and outlet port 14. In the chamber 10 is placed one or more succeptor 16 on which substrate crystals 18 are mounted. A conventional method of silicon deposition by using pyrolysis of silane is described hereinbelow. The conventional method is performed by admitting gaseous silane carried by a carrier hydrogen gas to the feed tube 12 of the chamber 10, as indicated by an arrow A. The gaseous silane carried by hydrogen gas enters via the tube 12 the chamber 10 as indicated by arrows B. Since the substrate 16 is heated to a high temperature for example l,l50C, the silane gas tion (I) is expressed as:

where P and P respectively represent partial pressure of the hydrogen gas and the silane gas when equilibrium established.

It should be noted that since not only the silane gas but the carrier hydrogen gas are introduced into the reaction chamber 10, the partial pressure of the hydrogen gas in the chamber 10 increases so that the rightward shift of equilibrium of the reaction (1) is undesirably suppressed.

In order to solve this problem, a method according to this invention employs as the carrier gas for the silane thus entered the reaction 10 thermally decomposes through the following reaction:

Sil-l4 (glfi-Si (s) 2H, (g)

The solid silicon produced through the above reaction deposits on the surface of the substrate 16 and, namely, a silicon wafer epitaxially grows on the surface of the substrate 16.

FIG. 2 is a graph showing the variation of the decom position rate a of the gaseous silane in the process of thereaction (1) against temperature of the substrate T in terms of ratios in gram molecule of the silane gas to the carrier gas, wherein absicissa and ordinate axes gas, a mixture of hydrogen gas and an inert gas such as argon, helium, neon, krypton, xenon, radon or nitrogen gas When the silane gas is carried by such a carrier gas as above mentioned, the partial pressure of hydrogen in the carrier gas introduced into the reaction chamber is considerably smaller whereby the decomposition of the silane gas is preferably promoted even at a relatively low temperature. The method according to this invention may therefore be performed at such a low temperature as not to affect the property of the resultant silicon wafer.

FIGS 3 and 6 respectively show graphs of the variation of the decomposition rate a against the temperature T of the substrate in terms of the ratio A when the carrier gas includes an inert gas in a ratio B in gram molecule of an inert gas to hydrogen gas. The proportions Aand B are varied from 0 to l and from 0 to 10, respectively.

It can be seen from FIGS. 3 and 6 that the larger the proportion B becomes, the more readilythedecompo sition of silane gasltakes place. For instance, when the proportions A and B areequal to 0.001 and, 10, respectively, the decomposition of silane gas sufficiently arises even when the substrate is at such a low temperature as 850C.

It can be also seen from the figures that in the process of the present method the substrate may be heated at from 450C to 1,300C. However, it is preferable to keep the temperatureof the substrate as high as possible to obtain a desired monocrystalline. When, on the other hand, it is required to prevent the vapour nucliation of the silane gas, the temperature of the substrate should be kept as low as possible. Therefore the optimum temperature of the substrate is about 900C when the ratios A and B are 0.00] and l0 ,'respectively.

FIG. 7 is a graphic illustration showing variations of the growth rate of silicon crystals, wherein a curve C represents the variation when the carrier gas made of only hydrogen gas is fed at a feed rate 40 (I/min.). Curve D represents the variation when the argon and hydrogen gases are equally fed at a rate of 20 (l/min.), respectively, and curve E, 18 and 2 (l/min.), respectively. These measurements were conducted at a feed rate of 40 (cc/min.) of silane gas.

It is apparent from the graph discussed above that growth temperature of silicon crystals becomes low as the mole ratio of argon in the carrier gas increases.

According to this invention, the substrate on which an epitaxial layer is to be formed is kept at a relatively low temperature such as 900C whereby the resultant silicon layer has a property suitable for highfrequency or low-noise transistor.

In addition, the method of this invention can be usable for growing silicon polycrystals on a suitable substrate such as silicon-oxide glass by keeping the temperature of the substrate as low as 650C, for example. The resultant silicon polycrystal layer has a property suitable for a low threshold MOS transistor.

The invention will be more fully understood with reference to the following illustrative specific embodiments:

EXAMPLE 1 lane employed is 40cc/min. Carrier gas is a mixture of hydrogen and argon and the feed rates of hydrogen and argon are 8l/min. and l2l/min., respectively.

Deposition of .epitaxial layer takes place on the substrate as a result of pyrolysis-of silane. The growth rate of the layer is about 0.5;L/min. X-ray diffraction patterns show that the deposited layer is a single crystal and has the same crystalline orientation as the substrate.

EXAMPLE H The same procedure outlined in Example I is repeated but the substrate of silicon single crystal is substituted for a surface oxidized silicon wafer. The substrate is heated to 700C. Gas feed rates of silane, hy-

drogen and argon are 20cc/min., 2l/min. and 8l/min., respectively. Polycrystal silicon layer of 2,000A in thickness is deposited by carrying out the reaction for about 10 minutes.

What is claimed is:

l. In a method of growing a silicon epitaxial layer on a substrate by pyrolysis of silane (SiHy) in an atmosphere containing hydrogen gas, the improvement comprising mixing a inert gas selected from the group consisting of argon, helium, neon, krypton, xenon, radon and nitrogen with said hydrogen gas at a gram molecule ratio of inert gas to hydrogen of up to 1,000 and heating said substrate to a temperature in the range from 450 to 700 C., whereby the partial pressure of the hydrogen may be controlled-so as to control the equilibrium of. the reaction toward formation of silicon. 

